1. Field of the Invention
The present invention relates to a semiconductor device carrying out switching control on a current flowing in the thickness direction of the semiconductor device. In particular, the present invention relates to a semiconductor device with a suppressed increase in turned-on resistance and an improved turn-off response.
2. Description of the Prior Art
Traditionally, a so-called isolated-gate bipolar transistor (IGBT), a transistor made by integrating bipolar and field-effect transistors into a single body, is used in applications where a high input impedance and a low output impedance are required.
FIG. 19 is a diagram showing the general structure of an IGBT. As shown in the figure, an ordinary IGBT has an n epitaxial layer 202 on a p+ silicon (Si) substrate 201. In addition, the IGBT also includes an n+ source region 206, a p body region 207 and a p+ body region 209 on the surface of the n epitaxial layer 202. A portion of the n epitaxial layer 202 other than the p body 207 is called an n drift region 202d. On the surface of the n epitaxial layer 202, there is a gate electrode 204 which is insulated from the n epitaxial layer 202 by a gate insulation film 203, and insulation films 205 and 208. The gate electrode 204 veils a portion of the n drift region 202d, a portion of the p body region 207 and a portion of the n+ source region 206 on the surface of the n epitaxial layer 202. In addition, on the front-surface side of the IGBT, there is provided a source electrode 210 which is conductive with respect to the n+ source region 206 and the p+ body region 209. On the back-surface side of the IGBT, on the other hand, there is provided a drain electrode 211 which is conductive with respect to the p+ substrate 201.
In the structure of the IGBT described above, the gate electrode 204, the n+ source region 206, the p body region 207 and the n drift region 202d constitute a field-effect transistor. To be more specific, the p body region 207 is the channel region and the n drift region 202d is the drain region. On the other hand, the p+ body region 209, the n drift region 202d and the p+ substrate 201 form a bipolar (pnp) transistor. To put it in detail, the p+ body region 209 is the collector, the n drift region 202d which also serves as the drain region of the field-effect transistor is the base of the bipolar transistor and the p+ substrate 201 is the emitter.
The main function of the IGBT with the configuration described above is to switch a current flowing from the drain electrode 211 to the source electrode 210 by controlling the voltage applied to the gate electrode 204. To put it in detail, with no voltage applied to the gate electrode 204, even if a voltage is applied to the IGBT so that the drain electrode 211 is set at a potential higher than the source electrode 210, no current flows from the drain electrode 211 to the source electrode 210 because the applied voltage is in a reverse direction with respect to a pn junction between the p body region 207 and the n drift region 202d as well as a pn junction between the p+ body region 209 and the n drift region 202d. If a positive voltage is applied to the gate 204 (with respect to the source electrode 210), however, an n channel is created on the surface of the p body region 207, putting the field-effect transistor in a turned-on state. In this state, electrons flow from the n+ source region 206 to the n drift region 202d by way of the n channel. Accordingly, the concentration of carriers (electrons in this case) in the n drift region 202d increases, reducing the resistance thereof. As a result, a diode formed by the n drift region 202d and the p+ substrate 201 conducts, causing holes to be injected from the p+ substrate 201 to the n drift region 202d. For this reason, the bipolar transistor is turned on, flowing a current from the drain electrode 211 to the source electrode 210 in the thickness (transversal) direction.
Here, when the positive voltage applied to the gate electrode 204 is cut off, the IGBT returns to the turned-off state. In the preceding turned-on state, however, the n drift region 202d was filled with both electrons and holes each at a high concentration. Thus, even when the positive voltage applied to the gate electrode 204 is removed, cutting off the injection of electrons from the n+ source region 206, the concentration of carriers in the n drift region 202d does not decrease immediately. As a result, the transient characteristic of the IGBT at a switch-off operation time indicates that the current does not decrease in magnitude immediately right after the switch-off operation as shown by a dashed line in a graph of FIG. 21. In this way, there is raised a problem of a long turning-off time and a conventional technology has been proposed to solve the problem, that is, to improve the turn-off characteristic of the IGBT.
Basically, according to a proposed means for shortening the turning-off time, a region for distributing recombination centers such as heavy-metal atoms and lattice defects at a high concentration is provided in the IGBT. Such recombination centers cause carriers to mutually extinguish each other so that the concentration of carriers causing the problem described above can be reduced at an early time. According to a technology disclosed in Japanese Published Unexamined Patent Application No. Sho 64-19771, for example, protons are radiated from the back-surface side of the IGBT (or the side of the p+ substrate 201 in the case of the IGBT shown FIG. 19) to distribute lattice defects over a narrow range in close proximity to the p+ substrate 201 inside the n drift region 202d. Refer to a distribution of concentration of lattice defects and impurities in a conventional semiconductor device shown in FIG. 20.
In the case of an IGBT with lattice defects distributed in a narrow range as is the case with the IGBT disclosed in Japanese Published Unexamined Patent Application No. Sho 64-19771, however, the reduction in turning-off time is extremely inadequate. This is because, in regions outside the narrow range, the reduction of the carrier concentration is slow. As a result, in the last portion of the turn-off characteristic, the convergence of the current is late as indicated by a solid line of a graph shown in FIG. 21. In addition, there is raised a problem that, since the lattice-defect distribution region is narrow, the location of the region may vary from device to device due to variations caused by manufacturing processes, greatly affecting the characteristics of the semiconductor device. This problem will arise even if the lattice-defect distribution region is provided in the p+ substrate 201. It should be noted that, instead of radiating ions such as protons as described above, an electron beam can be radiated to distribute lattice defects widely over the entire semiconductor device so as to adequately reduce the turning-off time as is disclosed in Japanese Published Unexamined Patent Application No. Hei 3-272184. In this case, however, lattice defects are also distributed in the portions serving as the field-effect transistor, causing the turned-on resistance to increase.
The present invention addresses the problems described above; it is thus an object of the present invention to provide a semiconductor device that has an adequately shortened turning-off time without an accompanying increase in turned-on resistance.
In order to achieve the object described above, according to an aspect of the present invention, there is provided a semiconductor device comprising:
a switching element provided on a surface of a semiconductor layer;
a substrate at another surface of the semiconductor layer;
a portion of the semiconductor layer located between the switching element and the substrate having an impurity concentration sufficient enough so that a region adjacent to the substrate is not depleted;
a defect region provided in a portion of said semiconductor layer includes an entire non depletion layer, wherein the non-depletion layer is not depleted after a switch-off operation;
a peak of lattice defect concentration within said non-depletion layer, wherein said lattice defect concentration in the non-depletion layer is sufficient to shorten lifetime of carriers and reduce turn-off time; and
a switching control having a current flowing in a thickness direction of the semiconductor layer when said switching element is turned on and off.
In the semiconductor described above, lattice defects are distributed in an entire portion of a semiconductor layer at a concentration higher than those of other portions and the entire portion is not depleted when the switching element is turned off from an turned-on state in which the switching element is turned on, causing a current to flow in the thickness (transversal) direction of the semiconductor layer. For this reason, in this entire portion, the life times of carriers are shortened. As a result, the concentration of carriers in this entire portion is decreased fast after the switching element is turned off, causing the current to converge to zero early. Since the concentration of lattice defects in the switching element is not in particular higher than those of other portions, on the other hand, the turned-on resistance is low, exhibiting an excellent characteristic of the on operation. Here, in many cases, the distribution of lattice defects in the semiconductor layer shows in actuality a continuously varying value of the concentration as is the case with Gauss"" distribution or Lorenz""s distribution. In such cases, the range forming a half-value width of the distribution is regarded as the defect region.
According to a desirable aspect of the present invention, there is provided a semiconductor device described above comprising a bipolar transistor with an emitter, a base and a collector thereof laid out in the thickness direction of the semiconductor layer wherein the switching element is a field-effect transistor which is turned on for injecting carriers to the bipolar transistor and the defect region includes an entire portion in the base in close proximity to the emitter which is not depleted after a switch-off operation.
The semiconductor described above is the so-called insulated-gate bipolar transistor (IGBT). In an turned-on state of the IGBT, carriers are injected to the base of the bipolar transistor from the field-effect transistor. Thus, the concentration of carriers in the base is increased, putting the bipolar transistor in an turned-on state and, hence, causing a current to flow. As the field-effect transistor is switched off, a depletion layer is spread from a pn junction between the base and the collector of the bipolar transistor. However, a region in the base in close proximity to a portion of the emitter which is not depleted is included in the defect region. Carriers in this region are extinguished fast, causing the current to converge to zero early. As a result, the IGBT exhibits an excellent turn-off response. Since the concentration of lattice defects in the field-effect transistor is not in particular higher than those of other portions, on the other hand, the turned-on resistance is low, exhibiting an excellent characteristic of the on operation.